1. Field of the Invention
The present invention relates to an optical pickup apparatus for reading and recording information on a recording medium (hereinafter referred to as “optical pickup apparatus” or “apparatus” as appropriate), more particularly to an optical pickup apparatus suffering reduced loss of reflected laser beam from a recording medium.
2. Description of the Related Art
Currently, in optical pickup apparatuses using light, such as a CD (compact disk) drive, information is read out in such a manner that a recording pit is produced by converging a laser beam emitted from a laser beam source, as micro spot, onto a track provided on a disk-like recording medium such as a CD, presence or absence of the pit is recorded as information, the micro spot is radiated on the track, and that the presence or absence of the pit on the track is detected by a reflected laser beam.
Recently, DVDs (digital versatile disks) having a recording capacity about 7 times as large as that of CDs have been widely used to meet the demand for increased recording capacity. Increase in recording capacity means improvement of the recording density, which depends upon the number of recording pits formed on a disk-like recording medium (hereinafter referred to as disk). In DVDs, decreasing the size of a recording pit, that is, decreasing the diameter of a laser beam spot on the disk is one key way for increasing the density. The size of the micro spot to be radiated on the disk is proportional to the wavelength of a laser beam and is inversely proportional to the numerical aperture of an objective lens. Accordingly, for decreasing the size of the recording pit, it is required to shorten the wavelength of the laser beam and/or to increase the numerical aperture of the objective lens.
However, DVDs are strongly required to be compatible with CDs from the viewpoint of backward comparability of software. Originally, an optical pickup apparatus was provided with one laser beam source for a laser beam with a wavelength of 635–650 nm and one objective lens having a numerical aperture of about 0.6 for DVDs, and also with another laser beam source for another laser beam with a wavelength of 780 nm and another objective lens having a numerical aperture of about 0.45 for CDs, thereby ensuring the compatibility between the both disks.
However, when the numerical aperture of the objective lens is increased, the convergence state of the laser beam deteriorates due to coma aberration with respect to the inclination of the optical disk. Since coma aberration is proportional to the third power of the numerical aperture of the objective lens and to the thickness of the disk substrate, DVDs are designed to have a disk substrate thickness of 0.6 mm, which is half that of CDs.
When the thickness of the substrate deviates from the prescribed value, spherical aberration occurs at a convergence position of light passing through the inward portion of the objective lens and a convergence position of light passing through the outward portion thereof. Therefore, when a CD is read using an objective lens having a numerical aperture of 0.6 optimized to the thickness of the DVD substrate, the spherical aberration must be corrected by limiting the outward portion of luminous flux incident on the lens or by slightly diverging the incident angle at the lens.
Thus, one objective lens adapted for the DVD may work compatibly for the CD with the necessary correction of spherical aberration, but two laser beam sources for laser beams with the above-mentioned respective different wavelengths have to be provided for compatibility with a write-once-read-many CD because the reflective recording layer of the CD is formed of an organic dye material and has a reflection coefficient as low as 6% for a laser beam having a wavelength of 635–650 nm appropriate to the DVD.
FIG. 5 is a block diagram of a first example of a conventional optical pickup apparatus. The first example includes laser beam sources 91 and 12 to emit laser beams with a wavelength of 650 nm for DVDs and a wavelength of 780 nm for CDs, respectively, and a wavelength selection prism 92 for reflecting/transmitting and making the respective laser beams travel along the same optical path. There is also provided a half mirror 11 for reflecting and guiding the laser beam to a collimating lens 13 and also for transmitting and guiding a reflected laser beam returning from a disk 18 to a photo-detector 90. The laser beam that has passed through the collimating lens 13 is reflected and guided by a reflecting mirror 15 to an objective lens 16 or 17, and then incident on the disk 18. The disk 18, either a DVD 18a or a CD 18b, is placed on a driving mechanism (not shown) and rotated thereby.
The objective lens 16 has a high numerical aperture (high NA) for DVD, and the objective lens 17 has a low numerical aperture (low NA) for CD. The two objective lenses can be switched over interchangeably for the DVD and the CD by a driving mechanism (not shown). The laser beam reflected at the disk 18 travels backward along the incoming path, passes through the half mirror 11, is received by the photo-detector 90, and converted thereby into an electrical signal.
FIGS. 6A to 6C are explanatory drawings of the wavelength selection prism 92. The wavelength selection prism 92 is provided with an optical path control film 80 having characteristics shown in FIG. 6C. The optical path control film 80 blocks a laser beam with a wavelength of 700 nm or less and transmits a laser beam with a wavelength of 750 nm or more. Accordingly, a laser beam 81 with a wavelength of 780 nm is not blocked by the optical path control film 80, and so travels straight therethrough as shown in FIG. 6A. A laser beam 82 with a wavelength of 650 nm from a direction perpendicular to the laser beam 81 is blocked and reflected by 90 degrees by the optical path control film 80, and takes the same optical path as the laser beam 81 with a wavelength of 780 nm as shown in FIG. 6B.
The apparatus shown in FIG. 5 operates as follows. A laser diode (wavelength: 650 nm) 91 for DVDs and another laser diode (wavelength: 780 nm) 12 for CDs, as light sources, are disposed orthogonal to each other so that respective laser beams emitted therefrom are guided into the same optical path by the wavelength selection prism 92. Then, the laser beam has its optical axis reflected by 90 degrees at the half mirror 11, and is converted into a parallel pencil by the collimating lens 13. The laser beam collimated is directed by the reflection mirror 15 toward the disk 18 with a recording layer, and is incident on an objective lens appropriately selected out of the objective lens 16 with a high NA for DVDs and the objective lens 17 with a low NA for CDs and switched over to be set in place by the drive mechanism (not shown). Specifically, when reading a DVD, the laser diode 91 for DVDs oscillates, and the objective lens 16 is set at the optical path to converge the laser beam onto the disk (DVD) 18a, and when reading a CD, the laser diode 12 for CDs oscillates and the objective lens 17 is set at the optical path to converge the laser beam onto the disk (CD) 18b. The above-described switching-over mechanism is incorporated into an axially-sliding-type actuator (not shown). The laser beam reflected at any of the disks starts traveling in the backward direction along the incoming path, passes through the half mirror 11, is incident on the photo-detector 90, and converted thereby into an electrical signal.
The apparatus of the first conventional example has the following disadvantage. In order to ensure compatibility among a DVD, a CD and a CD-R/RW (CD recordable/rewritable), two laser diodes having wavelengths different from each other are required, and a means for guiding two laser beams into the same optical path is additionally required.
In order to overcome the above disadvantage, the present inventors disclosed “Optical Pickup Apparatus for Reading and Recording Information on Recording Medium” in Japanese Patent Laid-open No. 2002-203334 (JP 2002-203334A). The apparatus, which is shown in FIG. 8 as a second conventional example, includes only one laser diode, and a super-resolution cut-off filter disposed immediately before one of two objective lenses, and is adapted to compatibly read and record information on two types of disks having respective recording densities different from each other.
The super-resolution cut-off filter is an optical filter for coherent light, realized using an art called super-resolution. Since the super-resolution is a well-known art which is described in detail in, for example, “Optical and Electro-Optical Engineering Contact”, Vol. 33, No. 11 (1995), a description thereof will be omitted.
The resolution of optical equipment is related to the diameter of a laser beam spot converged by an objective lens. As is well known, a formula of W=1.22λ/NA is valid, where W is the diameter of the main lobe of a convergent spot in paraxial approximation, λ is the wavelength of a laser beam, and NA is the numerical aperture. Accordingly, the wavelength λ has to be small and the numerical aperture NA has to be large in order to decrease the diameter W of the main lobe, that is, to obtain high resolution.
FIGS. 7A to 7D explain the relation between the configuration of the super-resolution cut-off filter and the diameter of the main lobe of a converged laser spot, in which FIGS. 7A and 7C show the configurations of the super-resolution cut-off filters as super-resolution cutting off means, and FIGS. 7B and 7D show the distribution of the diameters of the main lobes obtained by respective configurations. The diameter W of the main lobe, which is called a diffraction-limited spot diameter, is normally the minimum spot diameter realizable, and has a distribution as shown in FIG. 7B when a laser beam 50 is incident on the objective lens 16 shown in FIG. 7A. However, when a light-blocking plate 40 is provided immediately before the objective lens 16 as shown in FIG. 7C whereby the amplitude distribution at the lens aperture surface is transformed so as to be small at the inward portion and large at the outward portion, the diameter of the main lobe of the focus spot can be made smaller than the normal diffraction-limited value thereby realizing a distribution as shown in FIG. 7D.
Referring to FIG. 8, there are provided a laser diode (wavelength: 780 nm) 12 for a CD and a half mirror 11 which is a beam splitter element for reflecting and guiding a laser beam to a collimating lens 13 and also for transmitting and making a reflected laser beam from a disk 18 incident on a photo-detector 90 corresponding to the wavelength of the laser diode for a CD. There is also provided a reflecting mirror 15 for guiding the laser beam which passed through the collimating lens 13 to an objective lens 16 or 17, by which the laser beam is convergently incident on the disk 18. The disk 18, either a DVD 18a or a CD 18b, is placed on a driving mechanism (not shown) and rotated thereby. The objective lens 16 has a high numerical aperture (high NA) for a DVD, and the objective lens 17 has a low numerical aperture (low NA) for a CD. The laser beam that passed through the collimating lens 13 is guided to one of the objective lenses 16 and 17, which correspond to the DVD 18a and the CD 18b, respectively. A super-resolution cut-off filter 10 is disposed immediately before the objective lens 16 so as to be combined therewith. When the DVD 18a is read, the objective lens 16 combined with the super-resolution cut-off filter 10 is arranged at an optical path, and when the CD 18b is read, the objective lens 17 alone is arranged at the optical path. The objective lens 16 combined with the super-resolution cutoff filter 10, and the objective lens 17 are integrally structured with a switching-over means (not shown) for interchanging the two objective lenses 16 and 17 for the DVD 18a and the CD 18b, respectively, and with an actuator driving system (not shown) for controlling their position relative to a recording surface of the disk 18. The laser beam reflected at the disk 18 travels backward along the incoming path, passes through the half mirror 11, is received by a photo-detector 90, and converted thereby into an electrical signal. The super-resolution cut-off filter 10 comprises a light-transmissible plate glass 33, and two anti-reflection films 31 made of a multi-layer and formed on the both side surface of the plate glass 33 such that one is formed entirely on one side surface and the other is formed partly on the other side surface, specifically, formed at portions except a central portion provided with a metal reflection film 32 whereby super-resolution effect is produced.
The apparatus shown in FIG. 8 operates as follows. When recording and reading on the DVD 18a, a laser beam emitted from the laser diode (wavelength: 780 nm) 12 appropriate for a CD is reflected at the half mirror 11 and incident on the collimating lens 13. The laser beam has its diffusion angle collimated by the collimating lens 13, has its optical path changed by the reflecting mirror 15, passes through the super-resolution filter, is converged into a beam spot with a predetermined diameter by the objective lens 16, and incident on a recording surface of the DVD 18a. In this instance, it is supposed that the objective lens 16 combined with the super-resolution cut-off filter 10 is selected in advance and set in place by the driving mechanism (not shown). The laser beam reflected at the recording surface of the DVD 18a travels backward along the incoming optical path, passes through the objective lens 16 and the super-resolution cut-off filter 10, has its optical path changed by the reflecting mirror 15, is converged by the collimating lens 13, passes through the half mirror 11, is incident on the photodetector 90, and converted into an electrical signal. When information is recorded on the DVD 18a, the intensity of the laser beam is increased to a predetermined value, and when recorded information is read, the intensity of the laser beam is decreased to a predetermined value. This is controlled by a control circuit and a laser driving circuit (both circuits not shown). When reading the CD 18b, it is supposed that the objective lens 17 is selected in advance and set in place by the driving mechanism (not shown). And, when information recorded on the CD is read, the intensity of the laser beam is adjusted to a predetermined value by the control circuit and the laser driving circuit (both circuits not shown).
However, the apparatus according to the second conventional example shown in FIG. 8 has the following problems. The super-resolution cut-off filter 10 is disposed before the objective lens 16 so that information can be read and recorded on two types of disks having respective recording densities different from each other using only one laser diode appropriate for a CD having a wavelength of 780 nm. The laser beam reflected at the DVD 18a takes the incoming optical path backward, passes through the objective lens 16 and the super-resolution cut-off filter 10, is incident on the reflecting mirror 15, and finally is incident on the photo-detector 90, to be converted thereby into an electrical signal. The super-resolution cut-off filter 10 limits the quantity of the laser beam to be incident on the center of the objective lens 16 thereby decreasing the diameter of the main lobe of a focus spot to be smaller than an ordinary diffraction-limited value. As described above, the laser beam reflected at the DVD 18a passes again, now backward, through the super-resolution cut-off filter 10. Accordingly, the reflected laser beam has its quantity limited and decreased at the center, resulting in decreased quantity of the laser beam incident on the photo-detector 90.